Ion and electron beam lithographic systems utilize a stencil mask to pattern a resist coated target. These stencil masks can include a thin membrane which carries the mask pattern. Typically, the stencil mask includes a substrate, and the membrane is an area of reduced thickness on the substrate. The mask pattern can be formed by solid areas and openings in the membrane. The membrane can also include an ion absorbing layer formed of a conductive material deposited on the solid areas of the membrane. The conductivity of the ion absorbing layer can be used to provide a ground path to prevent charge build up on the membrane. In addition, the ion absorbing layer preferably has a high emissivity to facilitate radiative heat removal due to absorption of incident ions.
FIG. 1 schematically illustrates a prior art stencil mask 10. The stencil mask 10 includes a substrate 12 which has been etched to form a membrane 14. A mask pattern 16 has been formed on the membrane 14 by etching a desired pattern of openings 18 through the membrane 14. In addition, an ion absorbing layer 20 has been formed on one side of the membrane 14 on solid areas of the mask pattern 16. Typically, the substrate 12 and membrane 14 comprise silicon. Other deposited or grown materials, such as SiC, SiN, and BN, can also be used to form portions of the substrate 12 and membrane 14. A representative thickness for the membrane 14 is from 2-10 .mu.m.
One prior art process for forming a thin membrane is described in U.S. Pat. No. 5,110,373 entitled "Silicon Membrane With Controlled Stress". This process includes electrochemically etching a backside of a substrate with a required membrane pattern. A thickness of the membrane is controlled by initially doping the substrate to a required depth with dopants of a predetermined concentration. One aspect of prior art membrane formation processes is that the membrane can be difficult to uniformly etch with the required dimensional accuracy.
In addition, any layers of material added to portions of the membrane, such as the ion absorbing layer, can produce stresses and distortion in the membrane. For example, resist masks and hard masks, required for forming the ion absorbing layer, can stress the membrane. These stresses can distort the mask pattern, and contribute to membrane rupture. Some membrane fabrication processes may also require dry etching and handling of the thin membrane, which can lead to stressinduced distortion, as well as breakage of the membrane.
In view of the foregoing, improved methods are needed for fabricating stencil masks for electron beam and ion beam lithography. The present invention is directed to an improved method for fabricating stencil masks. The method is characterized by reduced complexity and an improved stencil mask.